The present invention relates to a pneumatic rubber tire having an integral innerliner of a rubber composition comprised of butyl rubber and/or halogenated butyl rubber and low molecular weight trans 1,4-polybutadiene rubber, wherein said rubber composition may also contain a minor amount of at least one additional, sulfur curable, elastomer.
U.S. Pat. No. 6,024,146 relates to a tire having an integral innerliner comprised of bromobutyl rubber, trans 1,4-polybutadiene rubber and, optionally, a minor amount of sulfur curable rubber.
Although the trans 1,4-polybutadiene rubber in U.S. Pat. No. 6,024,146 is broadly referred to, the specification specifically describes a high molecular weight trans 1,4-polybutadiene rubber having a preferable number average molecular weight value (Mn) measured by GPC (gel permeation chromatograph analysis) of greater than 130,000, although its first claim does not contain a molecular weight restriction. The specification observes that use of the high molecular weight trans 1,4-polybutadiene rubber in the tire innerliner rubber composition beneficially increases the green strength of the innerliner composition. Preparation of the trans 1,4-polybutadiene is recited as being prepared by batch polymerizing 1,3-butadiene in an organic solvent in the presence of cobalt octoate and triethyl aluminum as a catalyst system with a para alkyl substituted phenol as a catalyst modifier.
For this invention, a low molecular weight trans 1,4-polybutadiene rubber is prescribed having a number average molecular weight value (Mn) measured by GPC in a range of from 10,000 to 50,000 which is spaced apart from said high molecular weight trans 1,4-polybutadiene described in U.S. Pat. No. 6,024,146 by at least 80,000 molecular weight units and is thereby a significant and substantial departure from use of the aforesaid high molecular weight trans 1,4-polybutadiene rubber.
The low molecular weight trans 1,4-polybutadiene rubber for this invention may be prepared by polymerizing 1,3-butadiene monomer in an organic solvent (e.g. hexane) with a combination of triethylaluminum, barium thymolate and n-butyllithium (n-BuLi) as a catalyst complex system instead of the cobalt octoate and triethylaluminum based catalyst system utilized in said U.S. Pat. No. 6,024,146.
Historically, the inner surface of a pneumatic tire is typically comprised of a layer of a rubber composition designed to prevent or retard the permeation of air and moisture into the carcass from the tire""s inner air chamber. It is often referred to as an innerliner. Innerliners have also been used for many years in tubeless pneumatic vehicle tires to retard or prevent the escape of air used to inflate the tire, thereby maintaining tire pressure. Rubbers, such as butyl rubber and halogenated butyl rubber, often referred to as halobutyl rubber, as well as blends thereof, are often used for such tire innerliners which are relatively impermeable to air.
Historically, the tire innerliner itself is normally prepared by conventional calendering or milling techniques to form a strip of uncured rubber composition of appropriate width which is sometimes referred to as a gum strip. Typically, the gum strip is the first element of the tire applied to a tire building drum, over and around which the remainder of the tire is built. When the tire is cured, the innerliner becomes an integral, co-cured, part of the tire. Tire innerliners and their methods of preparation are well known to those having skill in such art.
The use of the prescribed low molecular weight trans 1,4-polybutadiene rubber for this invention is considered herein to be significant to reduce cost of an associated rubber composition for a tire innerliner where a portion of halobutyl rubber content is replaced, while significantly maintaining a low strain stiffness property and providing a desirable low air permeability. This is considered herein to be significant because, in contrast, while a relatively high molecular weight cis 1,4-polybutadiene rubber would be expected to similarly reduce rubber innerliner rubber composition cost, such higher molecular weight polybutadiene would be more difficult to process because of its higher Mooney viscosity and would have a much higher air permeability property.
In the description of this invention, the term xe2x80x9cphrxe2x80x9d means parts by weight of an ingredient per 100 parts by weight of elastomer in a rubber composition unless otherwise indicated. The terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d are used interchangeably unless otherwise indicated. The terms xe2x80x9ccurexe2x80x9d and xe2x80x9cvulcanizexe2x80x9d are used interchangeably unless otherwise indicated. The terms xe2x80x9crubber compositionxe2x80x9d and xe2x80x9crubber compoundxe2x80x9d are used interchangeably unless otherwise indicated.
In accordance with this invention a tire is provided having an innerliner rubber composition comprised of, based upon 100 parts of rubber (phr);
(A) from about 70 to about 98 phr of rubber selected from the group consisting of butyl rubber, chlorobutyl rubber, bromobutyl rubber and mixtures thereof;
(B) about 2 to 30 phr of a trans 1,4-polybutadiene rubber having a number average molecular weight (Mn) (as measured by GPC, or gel permeation chromatograph) of not greater than 50,000 and desirably in a range of from about 10,000 to about 50,000, a trans 1,4-content in a range of about 80 to about 85 percent, and a vinyl content of less than 5 percent;
(C) from zero to 30, alternatively from about 5 to about 15, phr of at least one elastomer selected from acrylonitrile/butadiene copolymer, styrene/butadiene copolymer, cis 1,4-polyisoprene natural and/or synthetic rubber and mixtures thereof.
A significant aspect of utilization of a low molecular weight trans 1,4-polybutadiene for the tire innerliner of this invention is improved processing with a substantially maintained low strain stiffness with a relatively small increase in air permeability.
While the trans 1,4-polybutadiene of aforesaid U.S. Pat. No. 6,024,146 is of a relatively high crystallinity, a significant aspect of this invention is the use of a low molecular weight trans 1,4-polybutadiene also of relatively high crystallinity to obtain the aforesaid rubber composition processing, physical property(ies) and suitable air permeability.
This is in contrast with using a low molecular weight trans 1,4-polybutadiene of relatively low or no crystallinity which would have much higher air permeability.
Thus a significant aspect of this invention is the combination of both low molecular weight and high crystallinity for the trans 1,4-polybutadiene for the tire innerliner rubber composition
A significance of relatively high crystallinity of the low molecular weight trans 1,4-polybutadiene is considered herein as being advantageous for maintaining low air permeability and low strain cured stiffness while providing improved processing.
The relatively high crystallinity aspect of the low molecular weight trans 1,4-polybutadiene, as measured by differential scanning calorimeter (DSC) at a heating rate of 10xc2x0 C. per minute, is evidenced by two relatively sharp melting point peaks, namely a first peak at about 36xc2x0 C. (about 30xc2x0 C. to about 40xc2x0 C.) and a second peak at about 44xc2x0 C. (about 40xc2x0 C. to about 50xc2x0 C.). It also has a Tg (glass transition temperature determined at a temperature rise of about 10xc2x0 C.) of about xe2x88x9291xc2x0 C. In contrast, the high molecular weight trans 1,4-polybutadiene has two melting point peaks with a first peak in a the range of 35xc2x0 C. to about 45xc2x0 C. and a second peak in a range of about 55xc2x0 C. to about 65xc2x0 C.
In general, the low molecular weight trans 1,4-polybutadiene polymer has a microstructure composed of a trans 1,4-content in a range of about 80 to about 85 percent, a vinyl 1,2-content in a range of about 2 to about 5 percent with the remainder being primarily of a cis 1,4-content in contrast to a vinyl content value of from 5 to 20 weight percent and a cis 1,4-content of from 2 to 15 weight percent of the high molecular weigh trans 1,4-polybutadiene polymer prescribed by U.S. Pat. No. 6,024,146.
Other than the presence of the required low molecular weight trans 1,4-polybutadiene, the remaining rubber components in the rubber compound for use as an innerliner may vary depending on the desired properties for the tire innerliner. For example, based on 100 parts by weight of total rubber, from about 70 to 98 phr is a xe2x80x9cbutyl-typexe2x80x9d rubber selected from the group consisting of butyl rubber, chlorobutyl rubber, bromobutyl rubber and mixtures thereof may be used. Desirably, the amount of xe2x80x9cbutyl-typexe2x80x9d rubber may range from about 85 to 95 phr. Usually, the desired xe2x80x9cbutyl-typexe2x80x9d rubber is brominated butyl rubber as bromobutyl rubber. In addition to the butyl type rubber, the rubber composition may contain from about zero to 30, alternately about 0 to about 15 phr of a non-butyl type, sulfur curable, elastomer selected from at least one of acrylonitrile/butadiene copolymer, styrene/butadiene copolymer, natural rubber and mixtures thereof A desirability of using a non-butyl type, sulfur curable, elastomer may vary, depending somewhat on the relative cost of the elastomer and the cured properties desired. A desirable non-butyl type rubber is considered herein to be an acrylonitrile/butadiene copolymer.
Butyl rubber is conventionally described as a copolymer of isobutylene and isoprene wherein the copolymer contains from about 2 to about 6 weight percent units derived from isoprene (and thus from about 94 to about 99 weight percent units derived from isobutylene). A halobutyl rubber is a butyl rubber which has been halogenated, usually with bromine or chlorine. Thus such halobutyl rubber is typically either bromobutyl rubber or chlorobutyl rubber. Such butyl rubber and halobutyl rubber are well known to those having skill in such art.
The rubber compound containing the low molecular weight trans 1,4-polybutadiene may be prepared by blending with various conventional rubber compounding ingredients, depending somewhat upon innerliner properties desired. Conventional ingredients commonly used in rubber vulcanizates are, for example, carbon black, tackifier resins, processing aids, talc, clay, mica, silica, antioxidants, antiozonants, stearic acid, activators, waxes, oils and peptizing agents. As known to those skilled in the art, depending on the intended use of the sulfur vulcanized rubber, certain additives mentioned above are commonly used in conventional amounts. Typical additions of carbon black comprise from about 10 to 100 parts by weight based on 100 parts by weight of rubber (phr), preferably 40 to 70 phr. Typical amounts of talc, clay, mica, silica and calcium carbonate may range from about 2 to 25 phr. Typical amounts of tackifier resins comprise about 2 to 10 phr. Typical amounts of processing aids comprise about 1 to 15 phr. Typical amounts of antioxidant comprise 1 to 5 phr. Typical amounts of stearic acid comprise 0.50 to about 2 phr. Typical amounts of zinc oxide comprise 1 to 5 phr. Typical amounts of oils comprise 2 to 30 phr. The presence and relative amounts of the above additives are not an aspect of the present invention.
The vulcanization of the compound for use as an innerliner is conducted in the presence of a sulfur vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric disulfide or sulfur olefin adducts. As known to those skilled in the art, sulfur vulcanizing agents are used in an amount ranging from about 0.2 to 5.0 phr with a range of from about 0.5 to 3.0 being preferred.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. A single accelerator system may be used, i.e., primary accelerator in conventional amounts ranging from about 0.5 to 3.0 phr. In the alternative, combinations of 2 or more accelerators may be used which may consist of a primary accelerator which is generally used in the larger amount (0.3 to 3.0 phr), and a secondary accelerator which is generally used in smaller amounts (0.05 to 10 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators have been known to produce a synergistic effect on the final properties and are somewhat better than those produced by either accelerator alone. In addition, delayed action accelerators may be used which are not effected by normal processing temperatures but produce satisfactory cures at ordinary vulcanization temperatures. Suitable types of accelerators that may be used are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamate and xanthates. Preferably, the primary accelerator is a disulfide or sulfenamide.
In practice the rubber compound is formed into a gum strip. As known to those skilled in the art, a gum strip is produced by a press or passing a rubber compound through a mill, calender, multi-head extruder or other suitable means. Preferably, the gum strip is produced by a calender because greater uniformity is believed to be provided. The uncured gum strip is then constructed as an inner surface (exposed inside surface) of an uncured rubber tire structure, also known as the carcass. The innerliner is then sulfur co-cured with the tire carcass during the tire curing operation under conditions of heat and pressure. The innerliner rubber gum strip may have a thickness, for example, in a range of about 0.04 to about 0.4 centimeters, depending somewhat upon the particular associated tire.
Vulcanization of the tire of the present invention is generally carried out at temperatures of between about 100xc2x0 C. and 200xc2x0 C. Usually the vulcanization is conducted at temperatures ranging from about 110xc2x0 C. to 180xc2x0 C. Any of the usual vulcanization processes may be used such as heating in a press or mold, heating with superheated steam or hot salt or in a salt bath. Preferably, the heating is accomplished in a press or mold in a method known to those skilled in the art of tire curing.
As a result of this vulcanization, the innerliner becomes an integral part of the tire by being co-cured therewith. The innerliner of the present invention, as mentioned above, may have an uncured gum thickness in the range of from about 0.04 to 0.4 centimeters. Preferably, the innerliner has an uncured gum thickness in the range of from about 0.08 to about 0.20 centimeters for passenger tires, although the innerliner can be considerably thicker for truck tire. For example, as a cured innerliner, the innerliner may have a thickness ranging from about 0.02 to about 0.35 centimeters with an innerliner thickness in a range of about 0.04 to about 0.15 cm for passenger tires and considerably thicker for truck tire and light truck tire applications.
The pneumatic tire with the integral innerliner may be constructed in the form of a passenger tire, truck tire, or other type of bias or radial pneumatic tire.
The following examples are presented in order to illustrate but not limit the present invention. The parts and percentages are by weight unless otherwise noted.